1. Field of the Invention
The invention pertains to a security feature in the form of a luminescent mark for incorporation into recording material and to heat-sensitive recording material into which the security feature of the proposed type has been incorporated.
2. Description of the Related Art
Recording materials are in an integral part of daily life in society and in the business world. Various solutions for proof-of-authenticity security features have already been proposed in the past for the use of recording materials which must be shown to be authorized for the purpose in question by a specific mark incorporated into the approved stock.
The authenticity of a document can be established passively by inspecting it for the presence of a watermark, for example. In its original meaning, a watermark is understood to be a mark in the paper, which is formed by differences in the thickness of the paper. A distinction is made between genuine watermarks, which are produced by displacing the pulp fibers (so-called line watermarks) or by concentrating the pulp fibers (so-called shadow watermarks) by the use of, for example, a dandy roll in the wire section of the papermaking machine; semi-genuine, so-called “molette” watermarks, which are produced by impressing a mark into the still-wet paper in the pressing section of the paper machine; and finally inauthentic or “pseudo” watermarks, which are usually produced outside the paper machine either by embossing or by printing the finished paper with a colorless lacquer, preferably one which is fluorescent under UV light. In the last-mentioned example of a “pseudo” watermark, of course, the addition of the mark to the paper produces no difference in the thickness of the paper.
In the prior art to be discussed here, DE 690 01 677 T2, for example, proposes the use of a synthetic printing carrier with pseudo-watermarks. The carrier consists of a substrate of plastic, at least one authentication or security symbol, preferably applied by gravure printing to change the opacity of the printing carrier, and at least one printable pigment coating, which covers the mark. The monochrome or polychrome mark should be almost invisible in reflected light but readily visible in transmitted light. The disadvantage of the known printing carrier is that the pseudo-watermark printed on it can be counterfeited relatively easily, which cannot be prevented by applying simple pigment coatings on top of it.
In principle, heat-sensitive recording materials with proof-of-authenticity security features in the form of inauthentic and fluorescent watermarks are also known. EP 0 844 097 A1, for example, discloses a latent image printed on the reverse of the recording material as a security feature for a heat-sensitive recording material. This image is produced by means of a security ink containing a fluorescent reagent. To form a second security feature in the form of a waterproof image on the reverse of the heat-sensitive recording material, the security ink contains a water-repelling agent. The security ink formulated in this way with the fluorescent reagent in the form of pigment or dye and with the water-repelling agent is contained or dispersed in an aqueous carrier, which, in addition to these components, can also contain a binder. The disadvantage of this proposal is that the water-repelling character of the security ink makes it more difficult to use the standard printing methods to preprint the carrier with multiple images and lettering, as is conventionally done.
Another fundamental disadvantage of the conventional pseudo-watermarks known from the state of the art—a disadvantage which also applies to the document discussed above—is to be found in the fact that they are applied afterwards to the finished paper by embossing or single-layer printing, for which reason it is relatively easy to counterfeit them.